Methods are known, for example irradiation therapy methods, which expose areas of a patient with doses of irradiation. Such therapy is usually preceded by an irradiation planning session in which the region and dosage of irradiation is determined. This may involve obtaining CT image data of the patient. These steps may be repeated to examine the progress of the therapy. To do this successive CT images of the patient are generated, compared and analysed. Problems that may arise in this process include movements of patient, patient respiration, and size changes of relevant portions of the patient, in particular the tissue to be treated or tissue that is not to be exposed.
WO 2009/042952 A1 describes an irradiation method using deformable image registration. The method includes obtaining a first image, obtaining a second image, determining a deformation field using the first and second images, and determining a transformation matrix from the deformation field. The transformation matrix may be used to position the patient relative to the radiation source to compensate for rotation and translation movements of the patient between the first and second images.